Generalization of motor resonance during the observation of hand, mouth, and eye movements

Transcranial magnetic stimulation (TMS) of the motor cortex shows that hand action observation (AO) modulates corticospinal excitability (CSE). CSE modulation alternatively maps low-level kinematic characteristics or higher-level features, like object-directed action goals. However, action execution is achieved through the control of muscle synergies, consisting of coordinated patterns of muscular activity during natural movements, rather than single muscles or object-directed goals. This synergistic organization of action execution also underlies the ability to produce the same functional output (i.e., grasping an object) using different effectors. We hypothesize that motor system activation during AO may rely on similar principles. To investigate this issue, we recorded both hand CSE and TMS-evoked finger movements which provide a much more complete description of coordinated patterns of muscular activity. Subjects passively watched hand, mouth and eyelid opening or closing, which are performing non-object-directed (intransitive) actions. Hand and mouth share the same potential to grasp objects, whereas eyelid does not allow object-directed (transitive) actions. Hand CSE modulation generalized to all effectors, while TMS evoked finger movements only to mouth AO. Such dissociation suggests that the two techniques may have different sensitivities to fine motor modulations induced by AO. Differently from evoked movements, which are sensitive to the possibility to achieve object-directed action, CSE is generically modulated by "opening" vs. "closing" movements, independently of which effector was observed. We propose that motor activities during AO might exploit the same synergistic mechanisms shown for the neural control of movement and organized around a limited set of motor primitives.

Influence of imagined posture and imagery modality on corticospinal excitability

Silvio Ionta

Single pulse transcranial magnetic stimulation (TMS) was used to test the assumption that kinesthetic imagery of action is more 'motor' than visual imagery of action. We assessed corticospinal excitability during motor imagery of a thumb-palm opposition movement by recording potentials evoked by TMS from two hand muscles that would (opponens pollicis, OP, target) or would not (abductor digiti minimi, ADM, control) be activated during actual performance of the very same movement. Participants were asked to imagine the thumb-palm opposition movement while maintaining first person imagery that was either purely visual or predominately kinesthetic. The motor imagery task was performed in two conditions in which the imagined and the actual hand could be either congruent or incongruent. Facilitation of potentials recorded from OP was higher during imagery carried out in mentally congruent than incongruent postures. This effect was largely due to lack of excitability recorded during incongruent kinesthetic imagery, which was indistinguishable from baseline imagery of the static hand. All other conditions differed from static imagery regardless of position. No significant effects were found in a control muscle (ADM) thus indicating that the effect was not related to spatial coding. Subjective reports obtained after the experiment indicate that the results cannot be ascribed to qualitative differences in the imagery experienced. For relatively simple motor tasks requiring no 'expertise' we found no detectable difference in the motor cortex due to imagery modality.

Elsevier2005

Fully automated modulation of virtual-body ownership supresses fronto-parietal mu rhythm

Olaf Blanke, Nathaniel Evans

After continuously applying spatially and temporally congruent visuo-tactile stimulation to external, body-like objects, healthy humans feel the fake part to be the origin of the touch sensations and feel it to be their own. Incongruent stimulation nullifies both illusory changes. This Rubber Hand Illusion (RHI) is the most used paradigm to experimentally investigate healthy humans’ bodily self-consciousness and the extension of body ownership to foreign objects [1]. Standard RHI setups are limited by physical constraints and suffer the disadvantage of imprecise temporo-spatial stimulus control. Further, little is known about the neural and electrophysiological mechanisms associated with the crossmodal conflict and the induced changes in body ownership during the RHI. Here we recorded 64-channel electroencephalography (EEG) to investigate the electrophysiological changes while exposed to a carefully controlled, bilateral RHI induced by haptics and virtual reality (VR) technology. Methods Participants (n=8) sat at a table with a chin rest. Arms were placed (palms up) underneath the table to rest on the legs. A virtual scene depicting the real-world experimental scene was displayed on an immersive Head Mounted Display (HMD). We employed a 2x2 factorial design (Object, Stroking). Participants saw either two arms or two non-body cylinders (Object) projecting from their shoulders onto a virtual table, seeing the virtual arms or cylinder in a position ~20cm above their real arms. Visuo-tactile stimulation was provided by four vibration motors affixed to the palms of the (unseen) left and right hands and by animation of corresponding virtual motors on the virtual hands. Two visuo-tactile modes were defined (Stroking): 1) synchronous: virtual visual motors were shown to vibrate in temporal and spatial synchrony with the tactile vibration motors and 2) asynchronous: virtual visual motors were shown to vibrate with a temporal delay (100±50ms) and a randomly selected spatial direction with respect to the tactile vibration motor pattern. Two additional baseline conditions were recorded (no visual animation of the virtual motors and no tactile stimulation; no visual animation of the virtual motors with tactile stimulation). Subjective experience of the illusion was gauged by a questionnaire. 64-channel EEG was sampled at 2048Hz. Bipolar electrooculograms were recorded for later artifact removal. EEG analysis consisted of breaking 30s of spontaneous EEG data per condition into epochs of 2s. Epochs contaminated with eye blink artifacts or transient changes in electrode-to-scalp conductance were removed. Spectral power changes were computed in three frequency bands: alpha (8-13 Hz), beta (14-30 Hz) and gamma (30-100 Hz). Statistical analysis was performed at the scalp level and included a suprathreshold cluster permutation test to control for Type I errors. The generators of the significant scalp maps were localized with an inverse solution (sLORETA) [2]. Results Repeated ANOVA analysis of the questionnaire indicated a significant interaction between synchrony and question (F=1.65, p=0.03), and a main synchrony effect (F=40.29, p

2010

Multiscale body maps in the human brain

Michel Akselrod

A large number of brain regions are dedicated to processing information from the body in order to enable interactions with the environment. During my thesis, I studied the functional organization of brain networks involved in processing bodily information. From the processing of unimodal low-level features to the unique experience of being a unified entity residing in a physical body, the brain processes and integrates bodily information at many different stages. Using ultra high-field functional Magnetic Resonance Imaging (fMRI), I conducted four studies to map and characterize multiscale body representations in the human brain. The goals of my thesis were first to extend the actual knowledge about primary sensorimotor representations, and second to develop novel approaches to investigate more complex and integrated forms of body representations. In studies I and II, I first investigated how natural touch was represented in the three first cortical areas processing tactile information. I applied a mapping procedure to identify in each of these three areas the somatosensory representations of 24 different body parts on hands, feet and legs at the level of single subjects. Using fMRI and resting-state data, I combined classical statistical analyses with modern methods of network analysis to describe the functional properties of the formed network. In study III, I applied these methods to investigate primary somatosensory and motor representations in a rare population of patients. Following limb loss, the targeted muscle and sensory reinnervation (TMSR) procedure enables the intuitive control of a myoelectric prosthesis and creates an artificial map of referred touch on the reinnervated skin. I mapped the primary somatosensory and motor representations of phantom sensations and phantom movements in TMSR patients. I investigated whether sensorimotor training enabled via TMSR was associated with preserved somatosensory and motor representations compared to healthy controls and amputee patients without TMSR. Finally in study IV, I studied brain regions involved in the subjective body experience. Following specific manipulations of sensorimotor information, it is possible to let participants experience a fake or virtual hand as their own and to give them the sensation of being in control of this hand. Using MR-compatible robotics and virtual reality, I investigated the brain regions associated with the alteration of the sense of hand ownership and the sense of hand agency. The present work provides important findings and promising tools regarding the understanding of brain networks processing bodily information. In particular, understanding the functional interactions between primary unimodal cortices and networks contributing to subjective body experience is a necessity to promote modern approaches in the fields of neuroprosthetic and human-machine interactions.